<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="+" onclick="f(1)">
<input type="button" value="-" onclick="f(2)">
<input type="button" value="*" onclick="f(3)">
<input type="button" value="/" onclick="f(4)">
<h3>
    结果：<span></span>
</h3>
<script>
    let i1=document.querySelector("#i1");
    let i2=document.querySelector("#i2");
    let h3=document.querySelector("span");
    function f(num) {
        if (isNaN(i1.value)||isNaN(i2.value)){
            h3.innerText="输入错误";
        }
        switch (num){
            case 1:h3.innerText=i1.value*1+i2.value*1;break;
            case 2:h3.innerText=i1.value*1-i2.value*1;break;
            case 3:h3.innerText=i1.value*1*i2.value*1;break;
            case 4:h3.innerText=i1.value*1/i2.value*1;break;
        }
    }
</script>
</body>
</html>